Impedance measuring circuit, it&#39;s method, and capacitance measuring circuit

ABSTRACT

An electrostatic capacitance detection circuit  10  comprises a DC voltage generator  11 , an operational amplifier  14  of which non-inverting input terminal is connected to specific potential, an impedance converter  16 , a resistance (R 1 )  12  connected between the DC voltage generator  11  and an inverting input terminal of the operational amplifier  14 , a resistance (R 2 )  13  connected between the inverting input terminal of the operational amplifier  14  and an output terminal of the impedance converter  16 , and a capacitor  15  connected between an output terminal of the operational amplifier  14  and an input terminal of the impedance converter  16 . A capacitor to be detected  17  is connected between the input terminal of the impedance converter  16  and specific potential.

TECHNICAL FIELD

The present invention relates to a circuit and a method that detectimpedance and electrostatic capacitance, especially relates to thecircuit and the method that detect very small impedance and capacitancewith high accuracy.

BACKGROUND ART

As a prior art of an electrostatic capacitance detection circuit, thatdescribed in Japanese Laid-Open Patent Application H09-280806 gazettecan be cited. FIG. 1 is a circuit diagram that shows this electrostaticcapacitance detection circuit. In this detection circuit, a capacitivesensor 92 comprised of electrodes 90 and 91 is connected to an invertinginput terminal of an operational amplifier 95 via a signal line 93. Anda capacitor 96 is connected between an output terminal of thisoperational amplifier 95 and the said inverting input terminal, andfurther an AC voltage Vac is applied to a non-inverting input terminal.Also, the said signal line 93 is wrapped up by a shield line 94 andshielded electrically against disturbance noise. And this shield line 94is connected to the non-inverting input terminal of the operationalamplifier 95. Output voltage Vd is obtained from an output terminal ofthe said operational amplifier 95 via a transformer 97.

In this detection circuit, the inverting input terminal and thenon-inverting input terminal of the operational amplifier 95 are in animaginary short status, so that the signal line 93 connected to theinverting input terminal and the shied line 94 connected to thenon-inverting input terminal have the almost same potential. Thereby,the signal line 93 is guarded by the shield line 94, that is, straycapacitance between the signal line 93 and the shield line 94 iscanceled, and the output voltage Vd, which is unlikely to be affected bythe stray capacitance, can be obtained.

According to this kind of conventional art, when capacitance of thecapacitive sensor 92 is big to some extend, it is indeed possible toobtain accurate output voltage Vd, which is not affected by the straycapacitance between the signal line 93 and the shield line 94. However,when very small capacitance, which equals to or is less than an order ofseveral pF or fF (femtofarad), is detected, an error is increased.

Also, depending on a frequency of the AC voltage Vac applied, a subtledisplacement of a phase and amplitude consequently arises between thevoltage of the inverting input terminal and that of the non-invertinginput terminal, which are in the imaginary short status, due to atracking error in the operational amplifier 59, and thereby thedetection error becomes bigger.

On the other hand, for lightweight and small audio communication devicesrepresented by a mobile phone or the like, there has been a demand of acompact amplifier circuit that sensitively and faithfully transformssounds detected by a capacitive sensor such as a capacitor microphoneinto an electric signal. If it is possible to accurately detect verysmall capacitance that equals to or is less than several pF or fF and/orits change, a high performance microphone that can detect sounds with avery high level of sensitivity and fidelity is realized, and therebyperformance for picking up sounds by the audio communication devicessuch as a mobile phone will make rapid progress.

This invention is devised in view of the above-mentioned situation, andaims at providing an impedance detection circuit, an electrostaticcapacitance detection circuit and the like that are capable ofaccurately detecting very small capacitance change, and suitable todetect impedance, such as a capacitive sensor including a capacitormicrophone used for lightweight and compact audio communication devices.

DISCLOSURE OF INVENTION

In order to achieve above objectives, the electrostatic capacitancedetection circuit according to the present invention is an impedancedetection circuit that outputs a detection signal corresponding toimpedance of an impedance element to be detected, comprising: animpedance converter of which input impedance is high and outputimpedance is low; a first capacitive impedance element; a firstoperational amplifier; a DC voltage generator that applies DC voltage tothe first operational amplifier; and a signal output terminal that isconnected to an output of the first operational amplifier, wherein aninput terminal of the impedance converter is connected to one end of theimpedance element to be detected and one end of the first impedanceelement, the first impedance element and the impedance converter areincluded in a negative feedback loop of the first operational amplifier,and the impedance element to be detected and the impedance detectioncircuit are located adjacently.

Also, the electrostatic capacitance detection circuit according to thepresent invention is an impedance detection circuit that outputs adetection signal corresponding to impedance of an impedance element tobe detected, comprising: an impedance converter of which input impedanceis high and output impedance is low; a first capacitive impedanceelement; a first operational amplifier; a DC voltage generator thatapplies DC voltage to the first operational amplifier; and a signaloutput terminal that is connected to an output of the first operationalamplifier, wherein an input terminal of the impedance converter isconnected to one end of the impedance element to be detected and one endof the first impedance element, the first impedance element and theimpedance converter are included in a negative feedback loop of thefirst operational amplifier, and the impedance element to be detected,the first impedance element and the impedance converter are locatedclosely.

In this patent document, “closely” means that the stray capacitance ofthe signal line is in a situation where the capacitance does not exceedten times as much as a bigger value of either the capacitance value ofthe impedance to be detected or the capacitance value of the firstcapacitive impedance element. It was found through experiences that theelectrostatic capacitance detection circuit of the present invention canprevent its detection sensitivity from being highly deteriorated whenthe stray capacitance of the signal line is set to have a capacitancevalue that does not exceed ten times as much as the capacitance value ofthe element connected. This stray capacitance of the signal line ismeasurable if it is measured under a situation where the impedance to bedetected, the first impedance element and the impedance converter arenot connected to the signal line. In this patent document, a statuswhere an object is in contact with other object side by side under theabove condition for being closely is called as “adjacently”.

Also, an impedance detection method and an electrostatic capacitancedetection method according to the present invention is an impedancedetection method that outputs a detection signal corresponding toimpedance of an impedance element to be detected, comprising steps for:connecting a first capacitive impedance element between an outputterminal of an operational amplifier and an input terminal of animpedance converter; connecting the impedance element to be detectedbetween the input terminal of the impedance converter and specificpotential; applying DC voltage to one input terminal of the operationalamplifier via a resistance, and other input terminal of the operationalamplifier is connected to specific potential; outputting voltage thatcomes out at the output terminal of the operational amplifier as adetection signal; and connecting the impedance element to be detected,the impedance converter and the first impedance element closely.

As a specific example, the electrostatic capacitance detection circuitis structured to comprise a DC voltage generator, a operationalamplifier of which non-inverting input terminal is connected to specificpotential, an impedance converter, a resistance (R2) connected betweenan inverting input terminal of the operational amplifier and an outputterminal of the impedance converter, a capacitive impedance elementconnected between the output terminal of the operational amplifier andthe input terminal of the impedance converter. An impedance to bedetected is located adjacently to this impedance detection circuit, orset closely to the impedance detection circuit at a short distance thatdoes not make the stray capacitance of the signal line exceed ten timesas much as maximum capacitance of an element connected, and connectedbetween the input terminal and the specific potential. The specificpotential in the example here indicates either certain standardpotential, specific DC potential, ground potential or a floating status,whichever suitable is selected according to a style of an embodiment.Also, a resistance (R1) connected between the DC voltage generator andthe inverting input terminal of the operational amplifier may further beadded.

According to the above structure, a certain voltage is applied to theimpedance to be detected, most of electric current that flows throughthe impedance to be detected is further sent to the impedance element,and then a signal corresponding to the impedance of the impedance to bedetected is output from a signal output terminal.

A resistance may be connected in parallel with the impedance element.

Also, one end of the impedance to be detected and the input terminal ofthe impedance converter may be connected each other by a signal linecovered with a shielding material, and a guard voltage applying unitthat applies specific voltage to the shielding material may be added.Here, the specific potential means some discretional regular potential,preferably a ground, but it may be the same potential as that of thevoltage of the signal line. Actions of the circuit are stabilized by thespecific voltage applied to the shielding material.

The guard voltage applying unit is, for example, a unit that generates acertain voltage using output voltage of the DC voltage generator oroutput voltage of the impedance converter as an input, or a unit thatconnects the shielding material to the ground.

Also, the impedance converter may be made up of a voltage follower, or avoltage amplifier of which voltage gain is less than or more than 1.Then, when an input side of the impedance converter is made up of acircuit comprising MOSFET, the input impedance may further be enhanced.

As a practical application of the present invention, it is preferablethat the impedance to be detected is a capacitance type of sensor thatdetects a physical quantity according to a fluctuation in thecapacitance, that the electrostatic capacitance detection circuit as theimpedance detection circuit is formed on a printed circuit board or asilicon substrate, and that the capacitance type of sensor and the boardare fixed. As a further specific example, it is possible that acapacitor microphone is adopted as the impedance to be detected, thatthe electrostatic capacitance detection circuit is embodied by an IC,that the capacitor microphone and the IC are integrated into one and putin a shield box as a microphone used for a mobile phone or the like. Inthis case, the capacitor microphone and the IC are fixed adjacently andconnected with a conductive board, a wiring pattern, a wire bonding orthe like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a conventional electrostaticcapacitance detection circuit.

FIG. 2 is a circuit diagram of an electrostatic capacitance detectioncircuit according to a first embodiment of the present invention.

FIG. 3 shows a specific circuit example of an impedance converter in theelectrostatic capacitance detection circuit shown in FIG. 2.

FIG. 4 is a circuit diagram of an electrostatic capacitance detectioncircuit according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram related to a variation of the electrostaticcapacitance detection circuit shown in FIG. 4.

FIG. 6 shows specific circuit examples of a guard voltage applyingcircuit shown in FIGS. 4 and 5.

FIG. 7 is a diagram showing a practical example of the electrostaticcapacitance detection circuit of the present invention used for electricdevices (a cross section diagram of a microphone).

FIG. 8A is a plain diagram showing an external outline of the microphoneshown in FIG. 7.

FIG. 8B is a front view diagram showing the external outline of themicrophone shown in FIG. 7.

FIG. 8C is a bottom view diagram showing the external outline of themicrophone shown in FIG. 7.

FIG. 9 is a cross section diagram of other example of the microphone.

FIG. 10A is a plain diagram showing an external outline of themicrophone shown in FIG. 9.

FIG. 10B is a front view showing the external outline of the microphoneshown in FIG. 9.

FIG. 11 is a circuit diagram of the electrostatic capacitance detectioncircuit according to other embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The following provides a detailed explanation of embodiments of thepresent invention with reference to diagrams.

(First Embodiment)

FIG. 2 is a circuit diagram of an impedance detection circuit accordingto a first embodiment of the present invention. In this diagram, acapacitor to be detected 17 (i.e. a capacitance type sensor that detectsvarious types of physical quantities using a fluctuation in theelectrostatic capacitance Cs such as a capacitor microphone in thisexample.), which is a subject for detection as an impedance to bedetected, is connected to an electrostatic capacitance detection circuit10 as the impedance detection circuit.

This electrostatic capacitance detection circuit 10 comprises an DCvoltage generator 11 that generates AC voltage, a resistance (R1) 12, aresistance (R2) 13, an operational amplifier 14, an impedance element 15(a capacitor with capacitance Cf in this example) and an impedanceconverter 16, and outputs a detection signal (voltage V out)corresponding to a temporal change in electrostatic capacitance of thecapacitor 17 from a signal output terminal 20. The “temporal change”here includes changes with respect to a frequency or a pulse, gradualchanges, random changes with time, and so on, and it may not necessarilyindicate periodicity.

One end of the DC voltage generator 11 is connected to specificpotential (a ground in this example), and other end (an output terminal)of that generates a certain DC voltage Vin. The resistance (R1) 12 isconnected between the output terminal of the DC voltage generator 11 andan inverting input terminal of the operational amplifier 14. Theoperational amplifier 14 is a voltage amplifier of which input impedanceand open loop gain are extremely high, a non-inverting input terminalhere is connected to specific potential (the ground in this example),and the non-inverting input terminal and the inverting input terminalare in an imaginary short status. In a negative feedback loop of theoperational amplifier 14, which is from an output terminal to theinverting input terminal of the operational amplifier 14, the capacitor15, the impedance converter 16 and the resistance (R2) 13 are connectedin series in this order.

The impedance converter 16 is a voltage amplifier of which inputimpedance is extremely high, output impedance is extremely low, andvoltage gain is A times. An input terminal 21 of this impedanceconverter 16 is connected to one end of the capacitor 17, and other endof the capacitor 17 is connected to the specific potential (the groundin this example). An output terminal of the operational amplifier 14 isconnected to an output signal of this electrostatic capacitancedetection circuit 10, i.e. the signal output terminal 20 for outputtinga detection signal corresponding to a change in the capacitance value ofthe capacitor 17. A variable A indicated for A times or the like in thispatent document shows any real number other than zero.

Actions of the electrostatic capacitance detection circuit 10 structuredabove are as follows.

Regarding an inverting amplification circuit comprising the resistance(R1) 12, the resistance (R2) 13 and the operational amplifier 14 and thelike, both of the input terminals of the operational amplifier 14 are inthe imaginary short status and in the same potential (e.g. 0 V), theimpedance thereof is extremely high, and no electric current flowsthrough, so that the electric current passed through the resistance (R1)12 becomes Vin/R1. And, because all of the electric current is sent tothe resistance (R2) 13, the following expression becomes effective whenoutput voltage of the impedance converter 16 is V2.Vin/R 1=−V 2/R 2

When summarizing this, the output voltage V2 of the impedance converter16 can be expressed by the following expression.V2=−(R 2/R 1)·Vin  (Expression 1)

Also, because the voltage gain of the impedance converter 16 is A, theinput voltage V1 is as follows from a relationship between the inputvoltage (voltage of the input terminal 21) V1 and the output voltage(voltage of the output terminal 22) V2.V1=(1/A)·V2  (Expression 2)

By the way, when the capacitor 17 is a capacitor microphone or the like,the capacitance Cs thereof is changed by a frequency of sound input.Here, when an electric charge corresponding to the change, which is sentfrom the operational amplifier 14 to the capacitor 15, i.e. from thecapacitor 15 to the capacitor 17, is ΔQ (i.e. for a change in thecapacitance of the capacitor 17), all of the electric charge Q is sentto the capacitor 17 because the input impedance of the impedanceconverter 16 is extremely high, and it becomes V1=ΔQ/ΔCs. Therefore, ΔVout for the change in the voltage Vout of the detection signal, whichis output from the signal output terminal 20, is as follows.ΔVout=(ΔCs/Cf)·V1  (Expression 3)

-   -   When V2 is deleted from the above expressions 1 and 2, the        following expression is obtained.        V1=−(R 2/R 1)·(Vin/A)  (Expression 4)

When this V1 is assigned to the above expression 3, the followingexpression is obtained. $\begin{matrix}\begin{matrix}{{Vout} = {{{- \left( {1/{Cf}} \right)} \cdot \left( {{R2}/{R1}} \right) \cdot \left( {{Vin}/A} \right) \cdot \Delta}\quad{Cs}}} \\{= {{k \cdot \Delta}\quad{Cs}}}\end{matrix} & \left( {{Expression}\quad 5} \right)\end{matrix}$

WhereK=−(1/Cf)·(R 2/R 1)·(Vin/A)  (Expression 6)

That is, ΔVout for the change in the output voltage Vout of thedetection signal becomes a value in proportion to ΔCs for the change inthe capacitance Cs of the capacitor 17. Therefore, a signalcorresponding to the sound input to the capacitor microphone can beobtained by extracting only ΔVout, which is the DC component of thedetection signal output from this electrostatic capacitance detectioncircuit 10. Then, it is possible to largely amplify the net signalcorresponding to the sound (the voltage corresponding to ΔCs), and ahighly sensitive microphone can be realized.

A proportionality constant k shown in the above expression 6 does notcontain an item that depends on the frequency (the sound frequency), andis a fixed value. Therefore, this electrostatic capacitance detectioncircuit 10 does not depend on the sound frequency, and with a fixedgain, it outputs a faithful voltage signal corresponding to loudness ofthe sound. Here, actions of the capacitor 17 have been examined from aviewpoint of voltage. When it is analyzed from an angle of electriccurrent for better understanding, it is as follows.

Suppose the capacitance of the capacitor 17 has been changed temporallysuch as follows.Cs=Cd+ΔC sin ωct  (Expression 7)

Here, Cd is standard capacitance originally and basically held by thecapacitor 17, ΔC is a peak value of the change, and ω c is a frequencythat changes the capacitance being detected for the capacitor 17. Atthis point, the electric current that flows to the capacitor 17 is asfollows:

(Expression 8)

Because all of this electric current flows to the capacitor 15:

(Expression 9)

Here, since each of the electric current in the expressions 8 and 9 isidentical, (Expression 10)

From the expressions 1 and 2, the expression 10 can be expressed asfollows.

(Expression 11)

Again, the change of the capacitor 17 is supposed to be output.

Also, since this electrostatic capacitance detection circuit 10 isoperated by a DC drive (the DC voltage generator 11), it is steadilyfunctioned when compared with a case of an AC drive so that noise or thelike can be restrained. Furthermore, parts for the DC transmitter, etc.become unnecessary, and thereby size of the circuit can be reduced.

FIG. 3 is a specific circuit example of the impedance converter 16 inthe electrostatic capacitance detection circuit 10 shown in FIG. 2. FIG.3A shows a voltage follower 16 a using an operational amplifier 30. Aninverting input terminal and an output terminal of the operationalamplifier 30 are short-circuited. The impedance converter 16, of whichinput impedance is extremely high and voltage gain A becomes 1, can beobtained by making a non-inverting input terminal of this operationalamplifier 30 be an input of the impedance converter 16, and making theoutput terminal of the operational amplifier 30 be an output of theimpedance converter 16.

FIG. 3B shows a non-inverting amplifier circuit 16 b using anoperational amplifier 31. A resistance (R3) 32 is connected between aninverting input terminal of the operational amplifier 31 and specificpotential, and a feedback resistance (resistance (R4) 33) is connectedbetween the inverting input terminal and the output terminal of theoperational amplifier 31. The impedance converter 16, of which inputimpedance is extremely high and voltage gain A becomes (R3+R4)/R3, canbe obtained by making a non-inverting input terminal of this operationalamplifier 31 be an input of the impedance converter 16, and making theoutput terminal of the operational amplifier 31 be an output of theimpedance converter 16.

FIG. 3C shows a circuit 16 c where a buffer in CMOS structure is addedto an input side of the operational amplifier as shown in FIG. 3A or B.As illustrated in the diagram, N type MOSFET34 and P type MOSFET35 areconnected in series between negative and positive power supplies via aresistance, and an output of the buffer is connected to the input of theoperational amplifier 30 (or 31). The impedance converter 16, of whichinput impedance is extremely high, can be obtained by making the inputof this buffer be an input of the impedance converter 16, and making theoutput terminal of the operational amplifier be an output of theimpedance converter 16.

FIG. 3D shows a circuit 16 d that is like a buffer at the input side ofFIG. 3C. As illustrated in the diagram, N type MOSFET 34 and P typeMOSFET 35 are connected in series between the positive and negativepower supplies, and the output is made from a connection point of bothMOSFET.

FIG. 3E is a circuit where a non-inverting input of an operationalamplifier 32 is made to be an input of the impedance converter, and anoutput and the inverting input of the operational amplifier 32 areconnected each other via a resistance. As shown in FIGS. 3D and E,having such structure realizes the impedance converter 16 of which inputimpedance is extremely high.

According to an experiment related to the present invention, in theelectrostatic capacitance detection circuit shown in FIG. 2, forexample, if the stray capacitance of the signal line exceeds 200 pF whenoriginal electrostatic capacitance of Cs (an impedance to be detected: amicrophone in the present embodiment) is 20 pF, the detectionsensitivity becomes much worse. Also, when the said Cs is checked with afew other electrostatic capacitance values, their results tend to be thesame.

Additionally, both of the capacitance Cf, which is the first impedanceelement, and the capacitor Cs are a capacitance element connected to thesignal line in this circuit, so that the same result as above isexpected for calculation of both of the elements.

From these experimental results and experiences, it was found out thatgood detective sensitivity is secured when the impedance to be detected,the first impedance element and the impedance converter are locatedclosely in a way that the stray capacitance of the signal line does notexceed ten times as much as the capacitance value of the relevant Cs orCf.

(Second Embodiment)

Next, the following describes an impedance detection circuit accordingto a second embodiment of the present invention.

FIG. 4 is a circuit diagram of an electrostatic capacitance detectioncircuit 40 as the impedance detection circuit according to the secondembodiment of the present invention. This electrostatic capacitancedetection circuit 40 is equivalent to what a guarding function is addedto the electrostatic capacitance detection circuit 10 according to thefirst embodiment. That is, as a cable to connect the capacitor 17 withthe electrostatic capacitance detection circuit 40, a signal line 41 (acoaxial cable) covered with a shielding line 42 is used, andadditionally a guard voltage applying circuit 43 a for applying guardvoltage, of which potential is the same as that of the signal line 41,is added to the shielding line 42 of the coaxial cable.

By making the guard voltage applying circuit 43 a be connected betweenthe output terminal of the DC voltage generator 11 and the shieldingline 42, receiving the output voltage Vin of the DC voltage generator 11as an input, and amplifying the voltage (or dividing the voltage) with acertain pre-adjusted voltage gain, the guard voltage applying circuit 43a serves as a DC voltage amplifier that generates guard voltage, ofwhich potential is the same as that of the voltage of the signal line41, and that outputs and applies the guard voltage to the shielding line42. The voltage gain of this guard voltage applying circuit 43 a isspecifically V1/Vin, that is, as seen from the above expression 4,adjusted to (−R2/R1)·(1/A).

Having such structure keeps the signal line 41 and the shielding line 42in the same potential all the time, and cancels capacitance (straycapacitance) between them, so that any undesirable situation, which thestray capacitance is added up to the capacitance of the capacitor 17 asa measurement error, can be avoided, any disturbance noise mixed in thesignal line 41 can be shielded by the shielding line 42, and therebymore accurate and stable capacitance detection becomes possible.

A connection point of the guard voltage applying circuit 43 a thatapplies guard voltage to the shielding line 42 is not limited to an areabetween the DC voltage generator 11 and the shielding line 42 shown inFIG. 4, and it may be located to an area between the output terminal ofthe impedance converter 16 and the shielding line 42, for example, likethe electrostatic capacitance detection circuit 45 shown in FIG. 5. Atthis time, by receiving the output voltage V2 of the impedance converter16 as an input and amplifying it with a certain voltage gain (1/A), aguard voltage applying circuit 43 b (or 43 c) may be adjusted togenerate guard voltage V1 and apply the voltage to the shielding line42.

By the way, if the guard voltage applying circuit is only limited to DCapplying, the cancellation effect of the stray capacitance cannot beexpected. In such a case, it is effective to have ground connection insimple structure where any disturbance noise is not easily mixed in.

FIG. 6 shows specific circuit examples of the guard voltage applyingcircuits 43 a˜c indicated in FIGS. 4 and 5. The guard voltage applyingcircuit 43 a shown in FIG. 6A is an inverting amplification circuithaving its variable resistance as a feedback resistance. The abovevoltage gain can be obtained by adjusting the resistance value of thefeedback resistance, and the guard voltage, of which potential is thesame as that of the signal line 41, can be generated. The guard voltageapplying circuit 43 b shown in FIG. 6B is a non-inverting amplificationcircuit composed of two resistances and one operational amplifier. Theguard voltage applying circuit 43 c shown in FIG. 6C is a voltagefollower composed of two resistances and one operational amplifier. Theguard voltage, of which potential is the same as that of the signal line41 in FIG. 5, can also be generated by adjusting the values of theresistance in these FIGS. 6B and C.

In case of operational errors, tracking errors or the like, there is apossibility to reduce them when the gain A is set to 1. Therefore, A=1is preferable.

As a practical application of the electrostatic capacitance detectioncircuit of the present invention used for electric devices, it can beconsidered that the impedance to be detected is a sensor that detects aphysical quantity according to a fluctuation in the impedance, that theimpedance detection circuit is formed on a printed circuit board or asilicon substrate, and that the sensor and the board or the substrateare fixed and integrated into one. To be more specific, it is possiblethat a capacitor microphone is adopted as the impedance to be detected,that the electrostatic capacitance detection circuit is embodied by anIC, that the capacitor microphone and the IC are integrated into one andput in a shield box as a microphone used for a mobile phone or the like.

FIG. 7 is a diagram showing a practical example to use the electrostaticcapacitance detection circuit according to the first embodiment for anelectric device. Here, it shows a cross section diagram of a microphone50 used for a mobile phone or the like which comprises a capacitormicrophone and an electrostatic capacitance detection circuit that areintegrated into one. This microphone 50 comprises a lid cover 51 havinga sound hole 52, an oscillating film 53 that oscillates with sounds, aring 54 that fixes the oscillating film 53, a spacer 55 a, a fixedelectrode 56 set up against the oscillating film 53 via the spacer 55 a,an isolation board 55 b that supports the fixed electrode 56, an IC chip58 forming the electrostatic capacitance detection circuit according tothe above embodiment, which is fixed on a backside of the isolationboard 55 b, an IC package 59 that molds the IC chip 58, externalelectrodes 61 a and 61 b that are connected by the IC chip 58, a wirebonding, a contact hole, and the like.

The oscillating film 53, which is one side of the electrodes that formsthe capacitor, is connected to specific potential (a ground in thisexample), and the fixed electrode 56, which is the other side of theelectrodes, is connected to a circuit of the IC chip 58 via an electricconductor such as an aluminum board or a wire bonding. Capacitance and achange in the capacitance of the capacitor comprising the oscillatingfilm 53 and the fixed electrode 56 are detected by the electrostaticcapacitance detection circuit in the IC chip 58 located adjacently viathe isolation board 55 b, transformed into an electric signal, andoutput from the external electrodes 61 a and 61 b, or the like. The lidcover 51, which is made from a metal such as aluminum, serves as a roleof a shield box that shields any disturbance noise mixed into the innercapacitors 53 and 56, and the IC chip 58 with a conductive film (notshown) formed on an upper surface of the isolation board 60. In thisexample, the fixed electrode 56 is connected to the circuit, and theoscillating film 53 is connected to specific potential. However, theoscillating film 53 may be connected to the circuit, and the fixedelectrode 56 may be connected to the specific potential. But, the formercase is preferable from past experiences.

FIG. 8 is an external view diagram showing an outline of the microphone50 shown in FIG. 7. FIG. 8A is a plain diagram, FIG. 8 B is a front viewdiagram, and FIG. 8C is a bottom view diagram. Size of the lid cover 51shown in FIG. 8A and FIG. 8B is, for example, approximately φ5 mm×2 mmin height. Four external electrodes 61 a˜61 d shown in FIG. 8C are, forexample, two terminals for a power supply and two terminals for anoutput signal of the electrostatic capacitance detection circuit.

In such a practical example, the capacitor to be detected (the capacitormicrophone in the example here) and the electrostatic capacitancedetection circuit (the IC chip in the example here) are locatedadjacently to be in contact side by side under the aforementionedcondition for being closely, and they are connected each other by anelectric conductor of which length is extremely short. Then, these partsare covered with a shield material such as a metal lid cover. Therefore,in the practical example like this, any negative impacts such asdisturbance noise, which is mixed into the signal line (the electricconductor) connecting the impedance to be detected and the electrostaticcapacitance detection circuit, can be ignored.

In this practical example, it is preferable that the capacitor to bedetected and the electrostatic capacitance detection circuit areconnected each other by a non-shielded (unshielded) conductive board,wiring pattern, wire bonding, lead line or the like through a shortestroute. That is, in this practical example, since it may be applied to acompact microphone where a shielding material is not used for its signalline, the capacitor to be detected and the electrostatic capacitancedetection circuit are connected each other by an extremely shortelectric conductor, and a special circuit for applying guard voltage tothe shield or the like is not located, so that the size of the circuitdoesn't get bigger and miniaturization of the circuit is not hindered.

As other example of the microphone, FIG. 9 and FIG. 10 show the circuitput on a board. They are basically the same as the one in FIG. 7 with anexception that the electrostatic capacitance detection circuit is put ona board 62.

When the second embodiment is applied to this practical example, thesize of the circuit gets a little bigger for a part of the shield of thesignal line. However, this structure may be used since this is ratherpreferable for more accurate measurement.

Although the impedance detection circuit and the electrostaticcapacitance detection circuit according to the present invention havebeen described based on the two embodiments, the present invention isnot limited to these embodiments.

For instance, in the second embodiment, though one layered shield cableis used as a cable to connect the capacitor 17 and the electrostaticcapacitance detection circuit 40, two layered shield cable can be usedin stead. In this case, the guard voltage is applied to its inner shieldcovering the signal line, and its outer shield covering the inner shieldis connected to the specific potential or the ground, so that ashielding effect against disturbance noise can be enhanced.

Also, as shown in FIG. 11, it is possible to connect a resistance 18 inparallel with the capacitor 15 in the electrostatic capacitancedetection circuits 10 and 30 according to the above embodiments. In thisway, a connecting point for the capacitor 15 and the capacitor 17 isconnected to the output terminal of the operational amplifier 14 via theresistance 18, so that having a floating status through a DC form can beavoided and the potential can be fixed.

Also, a device connected as the impedance to be connected is not limitedto a capacitor microphone and includes all of devices, which detectvarious physical quantities such as an acceleration sensor, aseismograph, a pressure sensor, a displacement sensor, a proximitysensor, a touch sensor, an ion sensor, a humidity sensor, a raindropsensor, a snow sensor, a thunder sensor, a placement sensor, a badcontact sensor, a configuration sensor, an endpoint detection sensor, anoscillation sensor, an ultrasonic wave sensor, an angular velocitysensor, a liquid quantity sensor, a gas sensor, an infrared rays sensor,a radiation sensor, a water gauge, a freeze sensor, a moisture meter, avibrometer, an electrification sensor, a publicly-known capacitive typesensor like a printed circuit board inspection device, or the like.

As has been clarified from the above explanation, by applying DC voltageto the operational amplifier and connecting the impedance to be detectedto the signal line, the impedance detection circuit, the electrostaticcapacitance detection device and their method according to the presentinvention detect impedance of the impedance to be detected. That is, thecapacitor is connected between the output terminal of the operationalamplifier, of which non-inverting input terminal is connected to thespecific potential, and the input terminal of the impedance converter,and further the impedance to be detected is connected between the inputterminal of the impedance converter and the specific potential.

In this way, most of electric charge sent to the impedance to bedetected flows to the impedance element, so that an accurate signalcorresponding to the impedance of the impedance to be detected is outputto the output terminal of the operational amplifier, which makes itpossible to detect very small capacitance. Especially, when each of theimpedance is capacitive, it is possible to detect very small capacitancethat equals to or is less than a fF order.

Then, because the non-inverting input terminal of the operationalamplifier is connected to the specific potential, and the DC voltage isapplied to the inverting input terminal via the resistance, theoperational amplifier is functioned steadily, and the noise mixed in thedetection signal is restrained. Also, as the entire detection circuit isoperated by the DC drive and does not require a DC signal transmitter orthe like, it can be simplified and miniaturized.

Also, since the capacitor is connected between the operational amplifierand the impedance converter, it does not cause a problem to degrade anS/N ratio due to thermal noise from the resistance when a resistance isconnected between the operational amplifier and the impedance converter.

In addition, by placing a circuit element connected to the signal lineclosely, or placing this impedance detection circuit adjacent to theimpedance to be detected, it becomes unnecessary to have a shield cableconnecting between them and a special circuit or the like that cancelsstray capacitance generated by the cable.

Because a change component, which corresponds to the change in theimpedance of the impedance to be detected, is generated at the outputterminal of the operational amplifier by extracting only the changecomponent of the output terminal, it realizes an operational amplifierthat is best suitable to a capacitive sensor of which capacitance ischanged according to the change in the physical quantity of thecapacitor microphone or the like. For example, a microphone, whichdetects sound with extremely high sensitivity, can be realized.

It is also possible to connect one end of the capacitor to be detectedand the input terminal of the impedance converter by the signal linecovered with a shielding material, and to add the guard voltage applyingunit that adds voltage of which potential is the same as that of thevoltage of the signal line. In this way, it makes it possible to guardthe signal line with the shield at the same potential, and to cancel thestray capacitance generated between the signal line and the shield, sothat minuter capacitance can be detected with high accuracy.

As has been mentioned, the present invention realizes an electrostaticcapacitance detection circuit or the like, which detects very smallimpedance and capacitance accurately, and is suitable forminiaturization, and especially sound performance of lightweight andcompact audio communication devices such as a mobile phone is rapidlyimproved and its practical value is extremely high.

INDUSTRIAL APPLICABILITY

The electrostatic capacitance detection circuit according to the presentinvention may be used as a detection circuit of a capacitance typesensor, especially as a microphone device that is equipped with compactand lightweight devices such as a mobile phone.

1. An impedance detection circuit that outputs a detection signalcorresponding to impedance of an impedance element to be detected,comprising: an impedance converter of which input impedance is high andoutput impedance is low; a first capacitive impedance element; a firstoperational amplifier; a DC voltage generator that applies DC voltage tothe first operational amplifier; and a signal output terminal that isconnected to an output of the first operational amplifier, wherein aninput terminal of the impedance converter is connected to one end of theimpedance element to be detected and one end of the first impedanceelement, the first impedance element and the impedance converter areincluded in a negative feedback loop of the first operational amplifier,and the impedance element to be detected and the impedance detectioncircuit are located adjacently.
 2. An impedance detection circuit thatoutputs a detection signal corresponding to impedance of an impedanceelement to be detected, comprising: an impedance converter of whichinput impedance is high and output impedance is low; a first capacitiveimpedance element; a first operational amplifier; a DC voltage generatorthat applies DC voltage to the first operational amplifier; and a signaloutput terminal that is connected to an output of the first operationalamplifier, wherein an input terminal of the impedance converter isconnected to one end of the impedance element to be detected and one endof the first impedance element, the first impedance element and theimpedance converter are included in a negative feedback loop of thefirst operational amplifier, and the impedance element to be detected,the first impedance element and the impedance converter are locatedclosely.
 3. The impedance detection circuit according to claim 1 or 2,wherein the impedance element to be detected is a capasitive impedanceelement.
 4. The impedance detection circuit according to any of claims 1through 3, further comprising a resistance element connected in parallelwith the first impedance element.
 5. The impedance detection circuitaccording to any of claims 1 through 4, further comprising a secondimpedance element connected between the DC voltage generator and thefirst operational amplifier.
 6. The impedance detection circuitaccording to any of claims 1 through 5, wherein the one end of theimpedance element to be detected and the input terminal of the impedanceconverter are connected each other by a signal line covered with ashielding material, and the impedance detection circuit further includesa guard voltage applying unit that applies specific voltage to theshielding material.
 7. The impedance detection circuit according toclaims 6, wherein the guard voltage applying unit receives outputvoltage of the DC voltage generator as an input.
 8. The impedancedetection circuit according to claims 6, wherein the guard voltageapplying unit receives output voltage of the impedance converter as aninput.
 9. The impedance detection circuit according to any of claims 1through 8, wherein the impedance converter is a voltage follower. 10.The impedance detection circuit according to any of claims 1 through 8,wherein the impedance converter is a voltage amplification circuit thatincludes a second operational amplifier and has a voltage gain biggerthan one.
 11. The impedance detection circuit according to any of claims1 through 8, wherein the impedance converter includes an input circuitcomposed of MOSFET and a second operational amplifier.
 12. Theelectrostatic capacitance detection circuit according to any of claims 1through 11, wherein the impedance element to be detected is a capacitivesensor of which capacitance is changed temporally, and the firstimpedance element is a capacitor.
 13. The electrostatic capacitancedetection circuit according to claim 12, wherein the impedance elementto be detected is a capacitor microphone.
 14. An impedance detectionmethod that outputs a detection signal corresponding to impedance of animpedance element to be detected, comprising steps for: connecting afirst capacitive impedance element between an output terminal of anoperational amplifier and an input terminal of an impedance converter;connecting the impedance element to be detected between the inputterminal of the impedance converter and specific potential; applying DCvoltage to one input terminal of the operational amplifier via aresistance, and other input terminal of the operational amplifier isconnected to specific potential; outputting voltage that comes out atthe output terminal of the operational amplifier as a detection signal;and connecting the impedance element to be detected, the impedanceconverter and the first impedance element closely.
 15. The impedancedetection method according to claims 14, wherein one end of theimpedance element to be detected and the input terminal of the impedanceconverter are connected each other by a signal line covered with ashielding material, and specific voltage is applied to the shieldingmaterial.